Devices and methods herein generally relate to xerographic or electrostatographic printing machines, and more particularly to a drum unit and an earth plate used in a drum unit, which are used in image forming apparatuses, such as copying machines, laser printers, and facsimile machine.
In an image forming apparatus, for example a copying machine, an original image or document is usually read by an exposure section to form an electrostatic latent image on a photosensitive drum. A developing unit for forming toner image is disposed about an outer circumference of the photosensitive drum. The developing unit develops the read image by charging toner supplied from a toner hopper so that it has an electrostatic charge that is opposite that of the electrostatic latent image on the photosensitive drum. The toner adheres to the photosensitive drum at oppositely charged portions of the drum corresponding to the electrostatic latent image through a developing sleeve.
Electrophotographic imaging members are well known in the art. One type of photoreceptor conventionally utilized for copiers and printers and the like comprises a hollow photosensitive drum. The drum includes a tube that is obtained by forming a conductive metal into a cylindrical shape. Typically, the drum has been dip coated with various coatings including at least one photosensitive coating. These photoreceptors are usually supported on an electrically conductive shaft by drum supporting hubs or end flanges. The hubs can be constructed of metal, which is very robust to fatigue effects and deformation due to interference with the drive shaft. Hubs provide excellent stability and efficient grounding over the part life, however these parts are not cost effective. Hubs made of plastic material are lower cost and have a hole through their center into which a supporting axle shaft is inserted. That is, a flange member is forced into openings at opposing ends of the drum tube to provide the interface to a driving mechanism for rotation of the drum. Hubs constructed of metal provide a direct grounding connection to the metal substrate, while hubs constructed of electrically insulating plastic material require an electrical grounding device, which can be secured to the hub and positioned to contact both the electrically conductive axle shaft and the electrically conductive metal substrate of the photosensitive drum.
To produce a conductive state between the drum tube and the apparatus body, an electrical grounding device, sometimes referred to herein as an earth plate, may be attached to the flange member. The earth plate has an outer peripheral contact portion that is configured to contact the drum tube. An inner contact portion of the earth plate is configured to be in contact with the periphery of the shaft pin.
Earth plate devices can be constructed of various metals including, aluminum, copper, bronze, stainless steel, and the like. Material selection is key to providing a good reliable spring contact with the shaft, to be robust against corrosion in various environments, and withstand frictional stress during machine operation. Earth plate design is critical to insure mechanical performance against fatigue failure induced by vibration or misalignment of the motor drive shaft, which provide a ground path for the photoreceptor. In some cases, where misalignment is extreme or external sub-systems are pushing against the drum unit, fatigue failure can occur, which can reduce or obstruct the grounding pathway and lead to print defects or potential disruption of the electrical system of the printer. The clip on the earth plate can be overly constrained, reducing the ability of the clips to freely flex, which may result in the tabs widening over time or fracturing and breaking off completely. Any intermittent loss of contact between the earth plate and the drive shaft will result in image defects on the print.
A need exists for an earth plate that is more robust and less susceptible to fatigue failure to extend drum unit life.